This invention relates to a class D amplifier circuit.
A class D amplifier circuit converts an input signal into a pulse width modulation signal of constant amplitude and amplifies power. For example, the class D amplifier circuit is used for power amplification of an audio signal. The class D amplifier circuit operates in binary and thus can drastically lessen the loss of a transistor. Further, the class D amplifier circuit has the advantage that it has higher efficiency than a linear amplifier regardless of whether the amplitude of an input signal is large or small.
The class D amplifier circuit includes, for example, an integration circuit for integrating input signals, a comparison circuit for making a comparison between an output signal of the integration circuit and a predetermined triangular wave signal, and a pulse width amplifier for outputting a pulse width modulation signal pulse-width-modulated based on the comparison circuit. The output signal of the pulse width amplifier is fed back into the input of the integration circuit. The output signal of the pulse width amplifier passes through a low-pass filter made up of a coil, a capacitor, and the like and becomes an analog signal for driving a load of a loudspeaker, etc.
As the class D amplifier circuits, for example, those as disclosed in Patent Documents 1 and 2 are known.
[Patent Document 1] JP-A-2006-42296
[Patent Document 2] U.S. Pat. No. 7,339,425
By the way, in such a class D amplifier circuit as described above, the low-pass filter becomes a factor of power loss. Also, the existence of the low-pass filter hinders miniaturization, slimming, cost reduction, etc., of the whole device. Hitherto, a class D amplifier circuit attempting to circumvent the loss and upsizing in the filter (refer to Patent Document 1) or a class D amplifier circuit requiring no filter (refer to Patent Document 2) has been provided. However, particularly the latter often involves various defects of occurrence of distortion, etc., and a completely ideal art is not yet provided.
For example, to realize filter-less configuration, a device having a sawtooth waveform with vertical change from a plus peak to a minus peak as the triangular wave signal may be used or operation of appropriately shifting the phase of the pulse width modulation signal as a differential signal or the like may be performed. However, according to the former, there is a fear of occurrence of distortion corresponding to a change portion with infinity of inclination; according to the latter, there is a need for worrying about occurrence of various defects caused by disorder of phase.